Measurement

Mackenzie-Lewis polygraph, London, England, 1919-1926

Measurements: doctors today, doctors in the past

Visit a doctor today and chances are that he or she will measure your blood pressure, your temperature, your height and weight, your cholesterol levels and many other things. But people didn’t always think that such ‘quantification’ could tell us anything interesting. Until the early modern period, people generally thought that numbers were useful for constructing machines and buildings, but didn’t feel that they could answer questions about the ‘how?’ and ‘why?’ of natural processes. When Isaac Newton published his work on gravity, many of his contemporaries pointed out that knowing how fast a body falls doesn't actually explain why it falls.

The invention of measuring instruments

Important measuring instruments such as the thermometer and the barometer were developed in the 1500s and 1600s, but they were very inaccurate. Despite this, some early modern doctors began to advocate the use of measurement for ‘medical mathematics’. Sanctorius Sanctorius (1561-1636), a physician at the University of Padua, was the first person to put a scale on a thermometer so it could register absolute numbers. He also invented a giant weighing machine. Sanctorius spent much of his adult life in this machine in a long-term self-experiment to determine, among other things, how much weight a human body loses through perspiration. William Harvey also used measurements in his experiments to prove the circulation of blood.

Enlightenment: the numbers debate

During the Enlightenment, scientists argued that numbers were more objective than qualitative descriptions. This argument had a great impact on medical research. Numbers became important both for observing individual patients and for observations on larger groups. Supporters of vaccination, for example, used statistics to show that it diminished the likelihood of catching smallpox.

Developments in measuring in the 1700s

Since the 1700s, physicians have developed ways to measure many features of the human body. We owe the second hand on watches, for instance, to the British physician Sir John Floyer (1649-1734), who used it to time his patients' pulse.

Inaccurate scientists

In the late 1800s, researchers became worried that the active involvement of the scientist could introduce inaccuracy and bias. With this in mind, technicians and scientists such as the French physiologist Etienne-Jules Marey developed ‘self-registering’ devices. These were instruments designed to produce measurements by themselves, such as Marey's sphygmograph for measuring blood pressure.

More measuring, less individuality

The quantification of the body changed the relationship between doctors and patients. For a long time, physicians had regarded each patient's case as unique. With quantification, scientists in the 1800s began to determine ‘normal’ ranges for physical features such as temperature and blood pressure. These numbers were supposed to be the same for every healthy person, allowing scientists such as Frenchman Adolphe Quetelet to develop statistical approaches to public health.